Organic Process Research & Development 2002, 6, 826−828
Technology Reports
Microwave-Assisted Synthesis of Room-Temperature Ionic Liquid Precursor in
Closed Vessel†
Bhushan M. Khadilkar and Geeta L. Rebeiro*
Organic Chemistry Laboratory, Applied Chemistry DiVision, UniVersity Institute of Chemical Technology,
UniVersity of Mumbai, Matunga, Mumbai 400 019, India
Scheme 1
Abstract:
We report here the synthesis of various alkylpyridinium and
1-alkyl-3-methylimidazolium halides on a large scale under
microwave irradiation, in a closed vessel. The reaction time was
drastically reduced as compared to conventional methods, and
good yields were obtained.
Introduction
rides, required for the widely used RTILs is quite time-
consuming. Conventionally it requires as long as 72 h of
reflux.13,14 Our aim was to reduce the overall time of
preparation of the ionic liquids and to synthesize the
precursor salts on a large scale in a shorter time period. While
we had been working on microwave-assisted synthesis of
these salts, we recently came across a report15 on the
preparation of alkyl-3-methylimidazolium salts under mi-
crowave irradiation. However, there are some major draw-
backs to this method. It is carried out in an open test tube.
The hygroscopic nature of salts may not permit a large-scale
preparation by this method. Also the irritant volatile alkyl
halides as well as the corrosive and hygroscopic 1-methyl-
imidazole are released inside the microwave cavity and
wasted. Heating volatile materials in an open vessel in a
microwave oven can be hazardous.
Room-temperature ionic liquid (RTIL) is no more a new
word to the scientific community today. The rising number
of publications is indicative of the potential of RTILs as
“neoteric solvents” for various chemical reactions. These
include Friedel-Crafts reactions,1-3 enzyme-catalyzed reac-
tions,4,5 hydrogenations,6,7 benzoylation,8 Heck reaction,9
Fischer indole synthesis,10 and so forth. RTILs are being
looked upon as future commercial11 solvents. The acidic ionic
liquids can act both as catalyst and as solvent. This dual
property of RTIL has turned out to be a boon in itself to
carry out a variety of chemical transformations and is aptly
given the name “designer solvent”. Most RTILs12 known
today consist of alkylpyridinium or dialkylimidazolium salts
which may be complexed with a Lewis acid for example,
AlCl3.
The preparation of some of these salts, for example,
1-butylpyridinium and 1-butyl-3-methylimidazolium chlo-
We report here the simple and quick method of prepara-
tion of alkylpyridinium and 1-alkyl-3-methylimidazolium
salts on a large scale in a closed vessel under microwave
irradiation in a CEM microwave digester, MARS 5.
* To whom correspondence should be addressed. E-mail: geeta_rebeiro@
hotmail.com.
† This work has been presented at the 5th International Electronic Conference
on Synthetic Organic Chemistry ECSOC-5, a symposium on MW-assisted
(1) Boon, J. A.; Levisky, J. A.; Pflug, J. L.; Wilkes, J. S. J. Org. Chem. 1986,
51, 480.
Results and Discussion
The quaternization (see Scheme 1) of some alkyl halides
such as BuBr and PrBr with 1-methylimidazole took place
under reflux conditions in a domestic microwave oven.
However, alkyl halides such as BuCl gave only a trace
amount of product formation, whereas 2-phenylethyl chlo-
ride, bromohexane, and 2,6-lutidine did not react under reflux
conditions under microwave irradiation at normal pressure.
When we carried out the reaction in a closed vessel in a
(2) Earle, M. J.; Adams, C. H.; Roberts, G.; Seddon, K. R. J. Chem. Soc. Chem.
Commun. 1998, 19, 2097.
(3) Song, C. E.; Shim, W. H.; Roh, E. J.; Chol, J. H. J. Chem. Soc., Chem.
Commun. 2000, 17, 1695.
(4) Schofer, S. H.; Kaftzik, N.; Wasserscheid, P.; Kragl, U. J. Chem. Soc.,
Chem. Commun. 2001, 5, 425.
(5) Freemantle, M. Chem. Ind. 2001, 1, 21.
(6) Saurez, P. A.; Dullius, J. E.; Einloft, S.; De Souza, R. F.; Dupont, J.
Polyhedron 1996, 15, 1217.
(7) Dyson, P. J.; Ellis, D. J.; Parker, D. G.; Welton, T. J. Chem. Soc., Chem.
Commun. 1999, 1, 25.
(8) Khadilkar, B. M.; Rebeiro, G. L. Synth. Commun. 2000, 30, 1605.
(9) Hisahiro, H.; Yumiko, S.; Takashi. H.; Toshio, S.; Masayoshi, A.; Keisuke,
O.; Chiaki, Y.; Tetrahedron Lett. 2001, 42, 4349.
(10) Khadilkar, B. M.; Rebeiro, G. L. Synthesis 2001, 3, 370.
(11) Chem. Ind. 2001, 8, 231.
(13) Carpio, R. A.; King, L. A.; Lindstrom, R. E.; Nardi, J. C.; Hussey, C. L.
J. Electrochem. Soc.: Electrochem. Sci. Technol. 1979, 126, 1644.
(14) Wilkes, J. S.; Levisky, J. A.; Robert, A. W.; Hussey, C. L. Inorg. Chem.
1982, 21, 1263.
(15) Varma, R. S.; Namboodiri, V. V. J. Chem. Soc.. Chem. Commun. 2001, 7,
643.
(12) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351.
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Vol. 6, No. 6, 2002 / Organic Process Research & Development
10.1021/op025551j CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/23/2002